Electric alarm system,preferably for a fire alarm



June 2, 1970 s s, MEYER 3,516,083

ELECTRIC ALARM SYSTEM, PREFERABLY FOR A FIRE ALARM Filed Dec. 2, 1965 4 rme/VS 5 Iva/v.0 Jena-64 Maya-e 8/ m wowf wt,

United States Patent 3,516,083 ELECTREC ALARM SYSTEM, PREFERABLY FOR A FIRE ALARM Svend Scheel Meyer, Herlev, near Copenhagen, Denmark,

asslgnor to Denske Securitas A/ S, Copenhagen, Denmark ]Filed Dec, 2, 1965, Ser. No. 511,081 Int. Cl. G08b 17/10 US. Cl. 340-237 6 Claims ABSTRACT OF THE DISCLOSURE An area monitoring smoke detecting electric alarm system having two ionizing chambers connected in series by a connecting conductor and subjected to radiation with one chamber being open to its surrounding environment and the other chamber being closed with a source of direct voltage being connected across the chambers and a voltage divider being connected by its outer terminals across said series connected chamber and by its tap to said connecting conductor and to one input terminal of an amplificr with the other input terminal of the amplifier being connected to the midpoint of the secondary coil of a transformer with the ends of the secondary coil being connected across the voltage divider which constitutes two branches of a bridge circuit wherein the output of the amplifier is connected to a signal relay through a rectifier and is further connected to the outer terminals of the primary coil of the transformer to consequently provide a positive feedback so that the amplifier is activated as a source of continuing alternating current when the ionizing current in the open ionizing chamber is changed by smoke particles entering the open ionizing chamber.

The invention relates to an electric alarm system, preferably for a fire alarm, and of the kind disclosed in US. Pat. No. 3,295,121, and consisting of two metallic ionizing chambers connected in series and each provided with a radioactive radiation member, one chamber being closed whereas the other chamber is open and communicates with a room to be supervised, and in which the walls of the two chambers are each connected to their separate pole of a voltage generator, and with a voltage-sensitive indicating member, which is connected to at least one electrode in the two chambers, and with a capacitative voltage-dependent potentiometer connected through its tap with an electrode projecting into the ionizing chambers, its outer points being each connected with their separate ionizing chamber, the potentiometer constituting the two branches of a bridge circuit fed with current partly from a source of direct current and partly from a source of alternating current, both connected across the outer point of the potentiometer, the voltage-sensitive indicating member, such as an amplifier connected with a signal relay mounted at a supervising centre and with a rectifier, is inserted between the tap of the potentiometer and a point of the alternating current source, which point is balanced in relation to the said outer points, the said alternating current source being screened against the direct current in the bridge circuit by means of capacitors.

Electric alarm systems of the aforesaid kind require, as will be seen from the foregoing, a source of alternating current which has to be connected to the alarm system as long as the latter is in operation. This increases the initial cost of the system and its cost of upkeep in so far as such a source of alternating current has to be inspected and checked and furthermore it may under adverse con ditions involve false alarms and in extreme cases failure of the alarm system. It is the object of the invention to devise the construction of an electric alarm system of the aforesaid kind having none of the said drawbacks, and

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an essential feature of an alarm system according to the invention consists in the modification that the outer point of the potentiometer are connected to the output termirials of the amplifier in such manner as to produce a positlve feedback circuit for the amplifier when the electric resistance in the open ionizing chamber of the system is increased.

The result is that when smoke particles or liquid particles entering the said ionizing chamber cause an increase in the internal leakage resistance of the chamber and consequently a variation in the direct current voltages across the normally balanced voltage-dependent capacitive potentiometer, the feed-back produced by the variation referred to will have the effect that the amplifier becomes self-oscillating and produces the alternating voltage necessary for energizing the signal relay so as to make an alarm signal.

In a suitable embodiment of the alarm system according to the invention and with the secondary winding of a transformer connected across the outer points of the potentiometer, the central point of the secondary winding constituting the point balanced in relation to the said outerpoints, the output terminals of the amplifier are connected in parallel to the primary winding of the transformer.

In another embodiment of the alarm system according to the invention the outer points of the potentiometer are furthermore connected in parallel with a short circuit and between the one outer point of the short circuit and one pole of the direct current source there is inserted a resistance element in such manner as to produce a false signal voltage across one branch of the bridge circuit when the short circuit is closed. As a result the false signal voltage will start the self-oscillation of the amplifier, thus producing the alternating current voltage necessary for exciting the signal relay making the alarm signal.

In a third embodiment of the alarm system according to the invention one outer point of the potentiometer is connected to one pole of the direct current source through a breaker contact adapted to break the direct current circuit through the potentiometer and to introduce a non-balance in the bridge circuit.

As a result, the breaker contact, which, for example, in the shape of a safety fuse may be arranged at a location especially exposed to tire risk, may start the selfoscillation of the amplifier so as to cause the alarm signal.

In a fourth embodiment of the alarm system according to the invention one outer point of the potentiometer is through a separate cable connected with a test key provided at the supervising centre, the said key being connected to one pole of the source of direct current in such a manner, that the said outer .point is impressed with an auxiliary voltage which introduces a nonbalance in the bridge circuit. As a result, the alarm system may be checked from the supervising centre solely by pressing the test key.

The invention will now be further described with reference to the drawing, in which:

FIG. 1 is a schematic representation of an embodiment of an electric alarm system according to the invention, and

FIG. 2 shows another embodiment of same.

The drawing shows schematically an electric alarm system of the kind consisting of two metallic ionizing chambers 1 and 2 connected in series, each provided with a radioactive radiation member, chamber 2 is closed while chamber 1 is open to its environmental surroundings is open and communicates with a room to be supervised for any particles of smoke or liquid occurring, the radioactive radiation member causing an ionization of the air in the two chambers, which results in leakage resistances between the outer terminals of the chambers. The chambers 1 and 2 are conventional and can be of the type shown in U.S. Pat. No. 3,295,121 or U.S. Pat, No. 2,702,898. The open chamber is connected to an outer terminal 3 and a central terminal 5 which is common for the two chambers, whilst the closed chamber is connected to the common central terminal 5 and another outer terminal 4. The terminals 3 and 4 are connected to a voltage-sensitive capacitative potentiometer 6, 7 by its two outer terminals 8 and 9, and the common central terminal 5 is connected to the central tap 10 of the potentiometer. The potentiometer 6, 7 constitutes one branch of an electric bridge circuit Which in the known installations of the aforesaid kind is fed with current from a source of alternating current through a transformer 12 that constitutes the other branch of the bridge circuit, and the indicating member consists of an alternating current amplifier 13 inserted between the tap 10 of the potentiometer 6, 7 and a point 14 of the secondary winding of the transformer 12, the said point being balanced in relation to the outer points 8, 9 of the potentiometer. The outer points 8, 9 of the potentiometer are according to the invention electrically connected to the output terminals of the amplifier 13, in such manner as to produce a positive feed-back circuit for the amplifier when the leakage resistance in chamber 1 is increased.

In one embodiment of the alarm system according to the invention of the output terminals of the amplifier 13 are connected in parallel to the primary winding 11 of the transformer 12. A source of direct current, such as a storage battery 15, has its positive pole connected through a high ionic resistor 16 to one outer terminal 9 of the potentiometer, whilst the negative pole of the battery, which pole may be earthed, is connected with the other outer terminal 8 of the potentiometer. The secondary winding of the transformer 12 is screened against direct current by means of capacitors 17 and 18. When the system is in operation a radiation source (not shown) is adjusted in such manner, that the ionizing current in the two ionizing chambers normally produces equal leakage resistances across chambers 1 and 2, and, in that case, the bridge circuit is in balance since the two capacitors 6 and 7 of the potentiometer are impressed with equal voltages. If particles of smoke or liquid enter the open ionizing chamber 1, the ionizing current in the said chamber will be reduced, whereby the voltage across the one capacitor 6 of the capacitative potentiometer increases and the voltage across the other capacitor 7 of the potentiometer drops. As a result, the capacity of the firstmentioned capacitor 6 is reduced and the capacity of the lastmentioned capacitor 7 is increased, so that the bridge circuit becomes unbalanced and a fault voltage occurs across the input terminals of the amplifier 13. Such part of the alarm system as has been described so far is arranged in or near the room to be supervised, Whereas a signal relay 20 inserted across a rectifier 19 in the output circuit of the amplifier 13 may be mounted at a supervising centre (not shown). When a fault voltage is produced across the input terminals of the amplifier 13, the said relay is fed with current and gives alarm to indicate particles of smoke or liquid entering the open ionizing chamber.

The contacts of the relay 20 may be connected to a visible signal member such as a lamp mounted at the supervising centre to indicate which of several installations has reacted to any particles of smoke or liquid, or to any fault. It will be appreciated, however, that both the relay 20 and the signal lamp may be provided near the ionizing chambers in case it is desired to have a signal in the room supervised to indicate that the alarm system in the said room has reacted.

In a modification of the alarm system as shown in FIG. 2 the outer points of the potentiometer are further according to the invention connected in parallel with a short circuit, and between one outer point 22 of the said short circuit and one pole of the direct current source 15 there is inserted a resistance element 23 in such manner that a voltage of direct current is produced across one branch 3, 5 of the bridge circuit when a short circuit contact is closed. The short circuit contact 21 may, for example, be adapted to be closed in case a fault occurs in the radioactive radiation member of the system, for example in case of damage to the radiation member. One outer point 3 of the potentiometer may in one embodiment of the plant according to the invention be connected to the one pole of the direct current source 15 through a breaker contact 24 which is adapted to break the direct current circuit through the potentiometer and thereby to introduce a non-balance in the bridge circuit. The breaker contact 24 may, for example in the form of a safety fuse, be arranged at a location especially exposed to danger of fire, and the breaker contact 24 may in the case be subjected to the action in known manner by a spring (not shown) which ensures that the contact is opened if the fuse is subjected to an increased, predetermined temperature, for example due to fire.

The one outer point of the potentiometer 3 may according to the invention be connected through a separate cable 26 to a test key 27, which is provided at the supervising centre and connected to the other pole of the source of direct current in such a manner that the said outer point 3 is impressed with an auxiliary voltage which introduces a direct current voltage across the potentiometer, whereby a non-balance in the bridge circuit is introduced. The said non-balance may, as shown in FIG. 2, be produced in connection with the resistor 23 inserted in the outer points 3 and the one pole of the direct current source 15 as a direct current voltage fed across a potentiometer which consists of two resistors 28 and 29.

It will be appreciated that both the amplifier 13 and the signal relay 20 may consist of any kind whatever of voltage sensitive and current-sensitive electric members such as transistors. It will also be appreciated that the potentiometer 6, 7 may be composed of several voltagedependent capacitors in case two single capacitors cannot stand up to the full direct current voltage and that the direct current voltage may be connected to the capacitors 6, 7 through balanced voltage-reducing potentiometers so as to obtain maximum voltage sensitivity for the said capacitors.

What I claim and desire to secure by Letters Patent is:

1. An area monitoring electric alarm system for detecting smoke or other environmental particles comprising two ionizing chambers each having two electrodes with said chambers being connected together in series by a common connecting conductor connecting an electrode of each chamber, one of said chambers having closed Walls to the environmental area to be monitored, the other chamber having perforated walls open to the environmental area to be monitored, a source of direct current connected across the non-connected electrodes of said chambers and to the outer terminals of a voltage divider in the form of a potentiometer having a center tap conductor means connecting said center tap to said common connecting conductor and to one input terminal of an amplifier, another input terminal on said amplifier, conductor means connecting said other input terminal of the amplifier to the center of a secondary coil of a transformer, a primary coil in said transformer, conductor means connecting the ends of said secondary coil across said outer terminals of said voltage divider, feed-back conductor means connecting the output of said amplifier to the primary coil of said transformer so that said amplifienbecomes self-oscillating when the current in the open 1onizing chamber is varied and indicating means operably connected to the output of said amplifier for providing a signal in response to oscillation of the output from said amplifier.

2. The invention of claim 1 wherein said output of said amplifier is connected to a relay forming a portion of indicating means through a rectifier.

3. The invention of claim 2 wherein said voltage divider is of the capacitative voltage-dependent type having two series connected capacitors with the center tap being connected between said capacitors.

4. The invention of claim 3 additionally including selectively operable circuit means for short-circuiting said ionization chamber and said voltage divider.

5. The invention of claim 3 additionally including fuse means between said source of direct current and said ionization chambers for breaking the circuit to said chambers in response to over-heating of said fuse means.

test circuit operable by a test key for creating an unbalance in the voltage divider for testing said alarm system.

References Cited UNITED STATES PATENTS 2,465,377 3/1949 Jaeger 340-237 XR 2,702,898 2/1955 Meili 340-237 2,809,317 10/1957 Meili 340-237 XR 10 JOHN W. CALDWELL, Primary Examiner D. K, MYER, Assistant Examiner US. Cl. X.R.

6. The invention of claim '1 additionally including a 15 250-44; 340-214, 228 

